1. Field of the Invention
The present invention relates to a polymeric membrane and, more particularly, to a polymeric membrane that is suitable for ultrafiltration, purification, concentration, dialysis and the related uses.
2. Description of Related Art
It is known that the genetic origin or the functional activity of cells can be determined and investigated by analyzing their nucleic acids. In fact, molecular biological analyses are applied to medical diagnosis, clinical diagnosis, evaluation of pharmaceutical compositions, food analysis, food production monitoring, cultivation of crops, cultivation of farm animals, and environmental analysis.
In a procedure of nucleic acids extraction, concentration and purification are required steps to obtain high-quality nucleic acids from biological materials. So far, the most useful way to concentrate and purify nucleic acids from crude samples is utilizing an ultrafiltration membrane. Ultrafiltration is a pressure-driven, convective process that separate species by molecular size and shape. Ultrafiltration membranes retain nucleic acid but are permeable by smaller components. However, nucleic acids in a biological sample often block the pores of the ultrafiltration membrane, resulting in decrease of efficiency of ultrafiltration.
Many kinds of membrane for isolating nucleic acids, as well as the related manufacturing process have been developed recently. For example, a process for cross-linking a cellulose hydrate membrane by the use of a water-soluble diepoxide in an alkaline solution is disclosed in U.S. Pat. No. 5,739,316. The membrane produced is highly resistant to alkalis, acids and cellulases. Also, a phase inversion process for the preparation of cellulose triacetate (CTA) and regenerated cellulose membranes is disclosed in U.S. Pat. No. 4,631,157. The membranes produced are durable, flexible, and chemically stable. However, the physical strength of these membranes is not strong enough, such that a supporting layer is required. Another method for making a microporous, skinless, and essentially symmetrical polyethersulfone membrane is disclosed in U.S. Pat. No. 6,056,903. The membrane produced in U.S. Pat. No. 6,056,903 demonstrated the advantages of an ultra-low binding rate of protein, a very high flow rate and the desired physical strength. However, for the application of concentration of nucleic acid, the sample recovering rate is relatively low compared to cellulose triacetate membranes, regenerated cellulose membranes, and cellulose acetate membranes.
For practical applications, a membrane is expected to have both acceptable bulk and surface properties. Nevertheless in most cases, an available membrane has either only desirable bulk properties (for example, mechanical strength, or solvent resistance) or surface properties (for example, water wettability, low protein absorbing tendency, thromboresistivity, controlled ion exchange capacity, or controlled surface chemical reactivity). For example, polyethersulfone (PES) membranes have appropriate mechanical strength. Nevertheless, the separation property of the PES membrane is poor. On the other hand, cellulose triacetate (CTA) is a preferable membrane material because of its ultra-high capacity for removing salts and recovering small molecules. However, the physical strength is weak.
Therefore, it would be beneficial to develop a method for making a membrane that has both desirable bulk and surface properties.
Making porous membranes from mixed polymers is a well-known technique, for example, U.S. Pat. No. 4,968,733 discloses the concept for making membranes from mixed polymers; the polymers include polyether-sulfone, polyvinylidene fluoride, polyerher-imide, polysulfone, polyethylene terephthalate, polyacrylonitrile, polymethyl methyacrylate and polycarbonate. An essential solvent, ε-caprolactom, is also a required element in the disclosure. However, no example in the reference demonstrates the best working conditions or results for making membranes from mixed polymers, for example, PES and CTA. Besides, the best mixing ratio of mixed polymers is not disclosed in the article. In the field of polymer membrane manufacture, the conditions for making membranes with different polymers mixed, especially from polymers with distinct properties, are difficult to deduce from previous experience. Here the working conditions are demonstrated, including the selection of solvent, non-solvent and additive for making PES/CTA mixed polymer membrane.